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                        <blockquote>
<p><a href="https://github.com/rust-random/book/blob/master/src/portability.md">portability.md</a><br />
commit: 2aaafc26d81b0df98c9af3edae8ab42fd9263c4e <br />
本章译文最后维护日期：2020-12-1</p>
</blockquote>
<h1><a class="header" href="#portability" id="portability">Portability</a></h1>
<h1><a class="header" href="#可移植性" id="可移植性">可移植性</a></h1>
<h2><a class="header" href="#definitions" id="definitions">Definitions</a></h2>
<h2><a class="header" href="#定义" id="定义">定义</a></h2>
<p>给定固定输入，所有 Rand 下的各种程序项（应）分为三类：</p>
<ul>
<li>输出是不确定的，因此永远不可重复</li>
<li>输出是确定的，但不被认为是可移植的</li>
<li>输出是确定的，被认为是可移植的</li>
</ul>
<p>一般来说，函数被认为是确定的和可移植的，<em>除非</em>它显然是非确定性的（例如<code>getrandom</code> 和 <code>ThreadRng</code>）<em>或者</em>它被特别标记为不可移植的（例如<code>StdRng</code>，<code>SmallRng</code>）。</p>
<h2><a class="header" href="#crate-versions" id="crate-versions">Crate versions</a></h2>
<h2><a class="header" href="#crate-的版本" id="crate-的版本">Crate 的版本</a></h2>
<p>我们尝试遵循<a href="https://docs.npmjs.com/misc/semver">语义化版本规则(semver rules)</a>中关于API的 <code>MAJOR.MINOR.PATCH</code>形式的版本管理规则：</p>
<ul>
<li>新的 *补丁(patch)*版本不应包括破坏API的更改或新增主要的新特性</li>
<li>在1.0版本之前，*小(minor)*版本可能包含破坏API的更改。1.0之后就不应该了。</li>
</ul>
<p>此外，我们还必须考虑<em>突破性变化</em>和<em>可移植性</em>。当给定固定输入时，</p>
<ul>
<li>对于非确定性程序项，实现可以在任何版本中更改</li>
<li>对于确定性的不可移植的程序项，在补丁(patch)版本中应该保留与原版本相同的输出，但在任何小(minor)版本中（包括1.0之后）可能会发生变化。</li>
<li>对于可移植性程序项，在跨越版本时出现对输出的任何更改都被视为等同于破坏API的更改。</li>
</ul>
<h3><a class="header" href="#testing" id="testing">Testing</a></h3>
<h3><a class="header" href="#测试" id="测试">测试</a></h3>
<p>如果可能的话，我们希望所有的伪随机算法都能检验其输出的值的稳定性：</p>
<ul>
<li>PRNG应与提供的参考值组成的向量进行比较 (<a href="https://github.com/rust-random/rngs/blob/master/rand_xoshiro/src/xoshiro256starstar.rs#L113">example</a>)</li>
<li>其他算法应该在 <code>value_stability</code>测试或类似的测试中包含它们自己的测试向量(<a href="https://github.com/rust-random/rand/blob/master/src/distributions/bernoulli.rs#L168">example</a>)</li>
</ul>
<h2><a class="header" href="#limitations" id="limitations">Limitations</a></h2>
<h2><a class="header" href="#局限性" id="局限性">局限性</a></h2>
<h3><a class="header" href="#portability-of-usize" id="portability-of-usize">Portability of usize</a></h3>
<h3><a class="header" href="#usize的可移植性" id="usize的可移植性">usize的可移植性</a></h3>
<p>遗憾的是，有一个不可移植的东西被烙进了 Rust语言的核心：<code>usize</code>（和 <code>isize</code>）。例如，空 <code>Vec</code> 的大小在32位和64位目标系统上会有所不同。在大多数情况下，这不是一个问题，但当涉及到以可移植的方式生成随机数时，这确实很重要。</p>
<p>一个简单的规则如下：如果需要可移植性，<em>永远不要</em>直接依赖 <code>usize</code> 或 <code>isize</code> 来抽样。</p>
<p>在 Rand 里，只要有可能，我们都遵守这条规则。所有需要一个有界的 <code>usize</code>值的序列相关的代码都会在 <code>u32</code>值上抽样，除非值的上限超过了 <code>u32::MAX</code>。
（请注意，在许多情况下，这实际上提高了基准性能。）</p>
<h3><a class="header" href="#portability-of-floats" id="portability-of-floats">Portability of floats</a></h3>
<h3><a class="header" href="#浮点数的可移植性" id="浮点数的可移植性">浮点数的可移植性</a></h3>
<p>浮点运算的结果取决于舍入模式和实现细节。尤其是超越函数的结果会因平台而异。因此，<code>rand_nudistr</code> 中的分布并不总是可移植到 <code>f32</code> 和 <code>f64</code> 中。然而，我们将努力使它们尽可能的可移植。</p>

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